Add support for user-specifiable name for cache directory

[herber]

Users should be able to choose the name of gittar's directory in ~/.

Instead of .gittar users can choose any name.

Example

gittar.fetch

gittar.fetch('lukeed/polka', { name: 'custom' }).then(console.log);
//=> ~/.custom/github/lukeed/polka/master.tar.gz

gittar.extract

gittar.extract('lukeed/polka#master', 'foo', { name: 'custom' });
// Searches in ~/.custom/ for the tarball
//=> contents: foo/polka-master/**

[lukeed]

Hey, thanks~!!

Left some comments for you.

Do you mind sharing your use case for this? IMO, it'd be better to have everything shared from .gittar explicitly so that users don't end up with copies of the same tarball in different root locations.

Of course, this pkg isn't that popular to really make that a concern, but if that ever were the case, I'd be afraid of having something existing in multiple locations, when the underlying gittar can know for certain if it has it. Also, current approach allows for easy cleanup -- single directory to remove.

[lukeed]

This could be:

function getTarFile(obj, name) {
  let dir = name ? path.join(HOME, `.${name}`) : DIR;
  return path.join(dir, obj.site, obj.repo, `${obj.type}.tar.gz`);
}

[lukeed]

Easier to read IMO

[lukeed]

This would have to be

opts = Object.assign({ strip:1, name:null }, opts, { file, cwd:dest });

Otherwise you're always overwriting a user-specified opts.name with name:undefined.

Used null here just for shorthand; that's not the problem in this case. :D

[herber]

Thanks for the suggestions!

I added this feature, because some users might worry about the .gittar folder, instead of .my-app. They probably don't know .gittar, and would just delete the folder.